<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .box {
      font-weight: 900;
      /* 默认不设置，或者设置700的比较多 */
    }
    /* 正常是normal,等于400 */
    /* bold是700，用的比较多 */
    .box1{
      font-weight:700;
    }
  
  </style>
</head>
<body>
  <div class="box">我是div元素</div>
  <div>我是div元素</div>
  <div class="box1">我是div元素</div>
</body>
</html>